In pressure measuring technology, a distinction is drawn between difference-, absolute- and relative pressure measuring sensors. Pressure difference sensors serve for measuring the difference between two different pressures. In the case of absolute pressure sensors, a pressure to be measured absolutely, i.e. as pressure difference relative to vacuum, is registered. With a relative pressure sensor, a pressure to be measured is registered in the form of a pressure difference relative to a reference pressure. The reference pressure is an ambient pressure, which rules, where the sensor is located. In the case of most applications, this is the atmospheric pressure at the location of use. Relative pressure sensors have usually a measuring chamber, which is sealed with a pressure sensitive, measuring membrane. On an outside of the measuring membrane, there acts, during operation, the pressure to be measured. On a side of the measuring membrane facing away, the chamber has an opening, through which the reference pressure acts in the interior of the chamber against the measuring membrane. Additionally, a measurement transmitter is provided, which converts a deflection of the measuring membrane dependent on the reference pressure and the pressure to be measured into an electrical, measured variable.
For pressure measuring apparatuses of the named types, it proves difficult to achieve a reliable seal between the pressure sensor element and the accommodating tube, without degrading the accuracy of the measurement of the sensor due to mechanical stresses.
It is known from the state of the art to use such pressure measuring transducers for hydrostatic level measurement (see FIG. 1). Among other things, such measuring devices, e.g. the Waterpilot FMX167, are sold by the Endress+Hauser.
These relative pressure sensors usually measure the difference between the pressure in a measured medium and the current atmospheric pressure. For relative pressure measurement, the reference air is led via a platform-side opening into the pressure chamber, and the surface of the measuring membrane facing away from the measuring chamber is supplied with the measured pressure. The resulting deformation of the measuring membrane is a measure for the relative pressure, which is converted in suitable manner into a measurement signal.
The mentioned supply of the reference air enables that moisture can get into the pressure chamber, which, in the case of subceeding, or falling beneath, the dew point, the moisture can condense in the interior of the sensor and degrade the functioning of the sensor. This is especially the case, when the surrounding air of the sensor has a higher temperature than the medium, whose pressure is to be measured.
Usually a ceramic cell is used as pressure measuring cell. The ceramic measuring cell is a dry measuring cell, i.e. the pressure acts directly on the robust ceramic membrane of the pressure measuring transducer and deflects it around max. 0.005 mm. On the electrodes of the ceramic substrate, a pressure-dependent capacitance change caused by the membrane movement is measured. The electronics then converts the capacitance change into a pressure proportional, measurement signal, which behaves linearly for fill level.
Because of the sensitive membrane, already small length changes of the mechanical components of the pressure measuring transducer, for example, length changes caused by temperature changes, affect the measurement signal and/or the mechanical components of the measuring cell. For example, stresses of the potting compound caused, for example, by temperature changes are transferred to the measurement transmitter electronics and, so, corrupt the measurement signal. In the worst case, electrical connections can be torn loose.